Recently, diagnoses and medical treatments using electronic endoscopes are commonly performed in the medical field. An electronic endoscope has a long insert section to be inserted into a patient's body cavity. The electronic endoscope incorporates an imaging section such as a CCD at its distal end. The electronic endoscope is connected to a light source apparatus. The light source apparatus delivers light to the electronic endoscope via a light guide such as an optical fiber. The electronic endoscope emits the light from its distal end to illuminate the body cavity. The imaging section at the distal end of the insert section captures an image of a tissue site in the body cavity while the body cavity is illuminated. The image is subjected to various processes in a processing apparatus connected to the electronic endoscope, and then displayed on a monitor. With the use of the electronic endoscope, a doctor can observe an image of the tissue site in the patient's body cavity real-time, which ensures accurate diagnosis.
The light source apparatus is a white light source, for example, a xenon lamp that emits white broadband light ranging from blue light to red light. An image (hereafter referred to as broadband image) captured with the illumination of the white broadband light shows the overall condition of the tissue site, for example, mucosa located on a surface of the tissue site, blood vessels inside the tissue, the shape and condition of the surface of the mucosa such as protrusion or depression, and the presence or absence of polyp.
In addition to the observation of the broadband image, observation with “special light”, that is, the observation using a narrow band light source that emits narrow band light in a specific wavelength range is performed as disclosed in Japanese Patent No. 3559755 (corresponding to U.S. Patent Application Publication No. 2003/0176768), for example. Light penetrates deeper into the tissue site as its wavelength increases, namely, blue light, green light, and red light increase in penetration depth in this order. Using the difference in the penetration depth, the endoscope system of Japanese Patent No. 3559755 generates a narrow band image in which blood vessels at a specific depth (for example, at the surface or at the depths) are enhanced. For example, with the emission of blue narrow band light having a small penetration depth, a narrow band image is generated with enhanced surface blood vessels. With the emission of green narrow band light having the penetration depth larger than that of the blue narrow band light, a narrow band image is generated with enhanced blood vessels located at the depth deeper than the surface. Thus, the blood vessels at a specific depth are more clearly observed in the narrow band image than in the broadband image.
To perform a diagnosis, it is preferable to observe both the narrow band image with enhanced blood vessel regions and the broadband image showing the overall condition of the tissue site. The endoscope system disclosed in Japanese Patent No. 3559755 captures an image using a spectral filter having a first area and a second area. The first area allows the broadband light emitted from the white light source to pass through it. The second area only allows the narrow band light to pass through it. The filter allows the simultaneous display of both the narrow band image and the broadband image. Additionally, the endoscope system of Japanese Patent No. 3559755 is provided with a mechanical drive mechanism to move the spectral filter such that one of the first and second areas of the spectral filter is inserted into the optical path of the white light source. Operating the drive mechanism moves the spectral filter to switch between the broadband light and the narrow band light.
The image sensor such as a CCD used in the electronic endoscope outputs image signals at 30 fps (frames per second) or 60 fps. Because the apparatus disclosed in Japanese Patent No. 3559755 mechanically moves a spectral filter to switch between the narrow band light and the broadband light, there is a time lag between obtaining an image signal for generating the narrow band image and obtaining an image signal for generating the broad band image. As a result, due to the motion of patient's body and the insert section during the time lag, misregistration between the narrow band image and the broadband image occurs.
To prevent the misregistration, a method using an acousto-optic filter is devised as disclosed in Japanese Patent Laid-Open Publication No. 2001-074555. With the application of vibration, the acousto-optic filter separates the narrow band light in a specific wavelength range from the broadband light such as white light. The acousto-optic filter is placed in front of the image sensor, so the light passes through the acousto-optic filter before it reaches the image sensor. Thereby, the acousto-optic filter separates the narrow band light in the specific wavelength range from the broadband light. However, the acousto-optic filter is expensive and increases cost.